Dear Jiang,
to draw a line or a dotted line is a question of graphics and
definitions of the specific software and has nothing to do with the
specific nature of a chemical bond.
"link" defines in your case only an additional chemical bond that is
not defined for the common polypeptide chain.
There was a paper/comment in Acta Cryst by Zbigniew Dauter "the
asymmetric unit should be close to the origin".
The uncomfortable source is most probably that the result of a
molecular replacement is sometimes an asymmetric unit "elsewhere" ,
origin shifts etc.
Nice weekend
Winfr
